Many natural gas reserves are highly contaminated by nitrogen, which has to be removed to meet pipeline specifications (nitrogen below 3%). Nitrogen rejection technology is also required in some chemical refineries where methane stream has to be purified before certain processes like steam reforming. Nitrogen removal from natural gas is a difficult and expensive process. It is estimated that nitrogen removal is more challenging and costly than corresponding carbon dioxide removal from coal bed methane. Currently there are four available nitrogen rejection technologies—cryogenic distillation, pressure swing adsorption (PSA), liquid-based absorption, and membrane separation (CH4-selective). Among them, only cryogenic distillation is a mature technology that has been commercially used on large scale. Although the cryogenic process has the highest methane recovery rate (about 98%), the complexity of pretreatment and high energy consumption make it less attractive for small throughputs. PSA and liquid absorption processes are better suited for small throughputs (<10 MMscfd) with relatively high nitrogen concentration. But both processes are capital intensive. Membrane technology is an attractive alternative compared to other separation technologies by virtue of its high adaptability, high reliability, low energy consumption, and low capital cost in installation, operation and maintenance. It utilizes the “free” well pressure as the driving force and separates enriched fast and slow gases into permeate and retentate streams respectively. With its modular nature and good space efficiency, membrane gas separation is especially attractive to small and remote sites.
The future growth of natural gas consumption and depletion of conventional resources will promote the development of low quality natural gas (LQNG) reserves, which account for almost one-third of known gas reserves in the US. Such LQNG contains excessive amounts of gas other than methane, mostly nitrogen, carbon dioxide, and hydrogen sulfide. It has been estimated that approximately 17% of the natural gas reserves in the US are sub-quality due to high levels of nitrogen (typically 5% to 40%). Moreover, considerable amount of nitrogen-rich natural gas (NRNG) reserves (with 10 to 85% N2) exist in many different regions in the world. Apart from the natural existence, nitrogen contamination can also be artificially introduced by nitrogen fracturing during some shale gas production processes. In such case, nitrogen content can reach as high as 50% at the start of work-up and then decline to 6-10%. The NRNG needs to be upgraded before it is sent to the commercial natural gas pipeline. The US natural gas pipeline specification typically limits nitrogen to be below 3%.